Background Evaluating premature (<65 years of age) mortality because of acute myocardial infarction (AMI) by demographic and regional characteristics may inform public health interventions. Methods and Results We used the Centers for Disease Control and Prevention's WONDER (Wide-Ranging Online Data for Epidemiologic Research) death certificate database to examine premature (<65 years of age) age-adjusted AMI mortality rates per 100 000 and average annual percentage change from 1999 to 2019. Overall, the age-adjusted AMI mortality rate was 13.4 (95% CI, 13.3-13.5). Middle-aged adults, men, non-Hispanic Black adults, and rural counties had higher mortality than young adults, women, NH White adults, and urban counties, respectively. Between 1999 and 2019, the age-adjusted AMI mortality rate decreased at an average annual percentage change of -3.4 per year (95% CI, -3.6 to -3.3), with the average annual percentage change showing higher decline in age-adjusted AMI mortality rates among large (-4.2 per year 95% CI, -4.4 to -4.0), and medium/small metros (-3.3 per year 95% CI, -3.5 to -3.1) than rural counties (-2.4 per year 95% CI, -2.8 to -1.9). Age-adjusted AMI mortality rates >90th percentile were distributed in the Southern states, and those with mortality <10th percentile were clustered in the Western and Northeastern states. After an initial decline between 1999 and 2011 (-4.3 per year 95% CI, -4.6 to -4.1), the average annual percentage change showed deceleration in mortality since 2011 (-2.1 per year 95% CI, -2.4 to -1.8). These trends were consistent across both sexes, all ethnicities and races, and urban/rural counties. Conclusions During the past 20 years, decline in premature AMI mortality has slowed down in the United States since 2011, with considerable heterogeneity across demographic groups, states, and urbanicity. Systemic efforts are mandated to address cardiovascular health disparities and outcomes among nonelderly adults.